Particle deposition apparatus and methods for forming nanostructures

ABSTRACT

A fast method of creating nanostructures comprising the steps of forming one or more electrically-charged regions ( 5 ) of predetermined shape on a surface ( 1 ) of a first material, by contacting the regions with a stamp for transferring electric charge, and providing electrically charged nanoparticles ( 7 ) of a second material, and permitting the particles to flow in the vicinity of the regions, to be deposited on the regions.

This invention relates to a method of forming structures of smalldimensions, for example of nanometer dimensions—commonly known asnanostructures—and also relates to methods involving interaction ofsmall particles, especially nanometer dimensioned particles withmaterial surfaces.

Hitherto, small-scale photonic or electronic devices have beenfabricated using photolithographic processing techniques. As sizes arereduced, it becomes difficult to form the individual geometric featuresof these devices at a sufficient degree of resolution due to the need toemploy radiation of ever-shorter wavelengths to expose the photoresist.

A process that presses a mould into a thin thermoplastic polymer film ona substrate to create vias and trenches with a minimum size of 25 nm isdisclosed in “Imprint of sub-25 nm vias and trenches in polymers” Chu etal, Applied Physics Letters 67(21), 20 Nov. 1995, pages 3114–3116.

Nanometer-sized metal and semiconductor particles (nanoparticles) may beregarded as potential components for photonic or quantum electronicdevices. Fabrication of these devices requires not only deposition butalso positioning of nanoparticles on a substrate. There are manydifferent ways of creating nanometer-scale structures using particles orclusters as building blocks, such as deposition from a suspension usingcapillary forces, which gives two- and three-dimensional arrays ofcrystal-like structures of particles.

Nanometer-scale chains of metal clusters have been fabricated with aresolution better than 200 nm. They nucleate at the boundary of thesubstrate and lines of photoresist during deposition ofcopper—“Microfabrication of nanoscale cluster chains on a patterned Sisurface”, Liu et al, Applied Physics Letters, 5 Oct. 1995, p 2030–2032.

“An arrangement of micrometer-sized powder particles by electron beamdrawing”, Fudouzi et al, Advanced Powder Technol., 1997, vol. 8, no. 3,pp 251–262, reports that electrically charged lines on the scale 20 μmmay be written in an insulating surface. It is shown that charged silicaspheres (5 μm diameter) in a suspension can be controllably directedtowards such charged lines.

On the topic of electrically charging surfaces, “Electrostatic writingand imaging using a force microscope” Saurenbach, IEEE Transactions onIndustry Applications, Volume 28 No. 1, January 1992, page 256 disclosesthe use of an electrostatic force microscope having a tungstenmicroscope tip, arranged to touch a polycarbonate surface with a smallvoltage to transfer charge to the surface in order to produce “chargespots” of micrometer dimensions.

“Charge storage on thin Sr Tr O₃ film by contact electrification”Uchiahashi et al, Japanese Journal of Applied Physics, Volume 33 (1994),pages 5573–5576 discloses charge storage on thin film by contactelectrification, by using an atomic force microscope. It was possible todiscriminate between charge dots spaced about 60 nm apart. The processis intended for non-volatile semiconductor memories.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improved methodby which devices having very small geometric features may be fabricated.

The concept of the present invention is to induce an electric charge invery small, as small as nanometric-dimensioned areas, on a surface,preferably by contacting a metallic tool in a controlled manner on aninsulating substrate. As a second step in the invention,nanometric-dimensioned particles in an aerosol or in liquid phase arethen influenced by the regions of electric charge on the substrate inorder to be deposited on the substrate or otherwise to interact with thesubstrate as explained below.

In a first aspect, the invention provides a method comprising the stepsof forming one or more electrically-charged regions of predeterminedshape on a surface of a first material, by contacting said regions witha tool means for transferring electric charge, and providing particlesof a second material, and permitting the particles to flow in thevicinity of said regions, to interact in a predetermined manner with theelectric charge of the said regions.

In a second aspect, the invention provides apparatus for carrying out amethod comprising tool means for contacting one or more regions ofpredetermined shape on a surface of a first material in order totransfer electric charge thereto, and means for permitting particles ofa second material to flow in the vicinity of said regions, to interactin a predetermined manner with said regions.

In a further aspect, the invention provides a method, comprising thesteps of forming one or more electrically-charged regions ofpredetermined shape on a surface of a material, providing particles ofnanometric dimensions, and permitting the particles to flow in thevicinity of said regions to interact in a predetermined manner with saidregions.

For the purposes of the present specification, “particles of nanometricdimensions” is intended to means particles having a diameter of 300nanometers or less. As preferred for most applications, the particlediameter is 50 nanometers or less, and as further preferred, in someapplications, for example optoelectronics, the particle diameter is 10nanometers or less.

The tool means may be a press or stamp having a contoured surface ofdimensions as large as millimeters or as small as nanometers, which isarranged to contact the surface of the substrate, and has aconfiguration conforming to the desired pattern or configuration ofelectric charge to be deposited on the substrate. The press or stamp maybe of a rigid material, or a resilient material, e.g. a metal coatedrubber material.

A significant advantage of employing a stamp is that a complexconfiguration of electrically charged regions of predetermined shape,extending over a wide area, may be formed in a single operation. Theprocess of the invention is therefore very much faster to carry out thanother methods, such as electron beam drawing or writing.

Alternatively the tool may take the form of a needle, rod or otherelongate object which is drawn across the surface in a desired path tocreate the desired pattern of electric charge. The tool may be the tipof a scanning probe microscope. The tool will usually be of metal butcan be of any other suitable rigid material having a work function whichis such in relation to the work function of the first material to permitcharge flow to the surface of the first material. The first material iscommonly an insulating material, but may be semiconducting or of anymaterial which is such as to hold the applied electric charge for asufficient length of time to permit the method of the invention to takeplace.

In addition to the locally charged regions, deposition of the secondmaterial may be assisted by application of an electrostaticprecipitation field.

Preferably, the particles of the second material have a secondelectrical charge of opposite sign to the first. Alternatively, theparticles of the second material may be of the same sign as that of thefirst electric charge, and the pattern of the deposited second materialis determined by the repulsion from the one or more electrically chargedregions.

The requirement that the particles be charged may in some cases berelaxed—particles may become polarised in an electric field and will beattracted towards electrostatically charged objects due to an electricfield gradient.

In another application, electrically neutral nanometric particles may beprojected against a surface, each to absorb one or more charge carriers,and to rebound from the substrate in an electrically charged condition.

As well as contact charging, other mechanisms may be employed for thecreation of locally-charged regions, including inducing a charge patternby irradiation with photons, e.g. by synchrotron light using a mask, orinducing a charge pattern by laser interference on a polar semiconductorsurface.

The particles of a second material may be formed by any suitableprocess. A preferred process of producing the particles in aerosol formis described below. Alternatively other processes such as laser ablationmay be employed.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention will be now be described merelyby way of example with reference to the accompanying drawings, in which:

FIG. 1 is a schematic diagram illustrating the method of the invention;

FIGS. 2 a to 2 c is a sequence illustrating the application ofelectrical charge to receptor regions of an insulating surface inaccordance with the invention;

FIG. 3 is a schematic view of a deposition chamber (precipitator) for anaerosol nanoparticle generator, for the method of the invention;

FIG. 4 illustrates, in diagrammatic form, an aerosol nanoparticlegenerator, described in our co-pending PCT Application No. GB98/03429;

FIGS. 5 a–c to 9 are scanning electron micrographs of surfaces ofmaterials having particulate deposits thereon, formed in accordance withthe invention; and

FIGS. 10–20 are schematic drawings showing various embodiments of thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the FIG. 1 of the drawings, the surface of a siliconwafer 3 is oxidised to produce a silicon dioxide layer 1, and localisedregions 5 of negative charge are imprinted on the surface. Nanoparticles7 formed in an aerosol unit are impressed with a positive charge and areattracted to the locally charged regions 5 of the silica surface layer,with the assistance of a local electric field F.

One method of applying the local charge to the surface is illustrated inFIG. 2 a to 2 c. A nanoprinting stamp 9 is made from a conductingmaterial (or from an insulator coated with a metal), and is brought intocontact with the insulating surface 1. The stamp 9 has protrusions 11formed on its contact surface in a predetermined configuration. Thewidth of these protrusions may range from dimensions of nanometers up tothe macroscopic millimeter range, preferably fabricated by electron beamlithography. The height of the protrusions is not material to the regiondefinition. After contact, localised charged regions are left on thesurface 1 of the substrate exactly mirroring the dimensions andstructures of the stamp protrusions.

The basis of this method is that charges cross the interface of aninsulator and a metal brought into contact. After the metal is removed,a charge is retained on the insulator. The sign and amount of chargetransferred depends approximately linearly on the work function or FreeEnergy of the metal in relation to the work function or Free Energy ofthe insulating substrate. The amount of charge may be increased byproviding a potential difference between the metal and the insulator. Itis estimated that, with the method of this preferred embodiment, 10⁵charges per square micrometer or less are transferred.

The substrate, which now has a pattern of charge on its surface 1, isplaced in a deposition chamber or precipitator for an aerosolnanoparticle generator as shown in FIG. 3. The generator producesparticles with a controlled charge, either positive or negative. If theaerosol particles have a polarity opposite to that of the charge on thesurface of the substrate, the particles will preferentially depositwhere the substrate is charged (FIG. 1), whereas particles with the samepolarity as the surface charge pattern will be repelled from thepattern, and are deposited in the spaces between the locally chargedregions. In the case of no applied electric field, particles withopposite charge states will still deposit where the substrate ischarged, whereas particles with the same polarity as the surface chargepattern will not be deposited.

Further processing steps may then be undertaken to fix the particlespermanently to the surface.

Referring to FIG. 3 there is shown a chamber 20 which is electricgrounded having an inlet 22 in its upper wall for receiving particles 7in the form of an aerosol. A electrode 24 in the chamber is connected toa source of potential 26 in order to generate an electric field betweenthe electrode and the walls of the chamber. The electrode 24 is mountedon an insulating tube 28. The electrically charged sample 1 is placed onthe upper surface of the electrode 24. The sample is a distance of somecentimeters from the opening 22.

In use, particles 23 entering the chamber through opening 22 flowtowards sample 1. The electric charge on the sample as shown in FIG. 1may be sufficient to attract the particles for deposition. However, asshown, the deposition may be assisted by the electric field existingbetween electrode 24 and the walls of chamber 20.

In this case, the particle deposition may take place (a) by attractingparticles of a different polarity to the charged regions and (b) bydeflecting particles from the charged regions. In the first caseparticles are deposited at the charged regions but are also deposited onareas in between the regions with lower density and randomly. This isdependent on the distances between the fields, the strength of themacroscopic electric field applied, the particle size, and the particlespeed in the gas flow. In the second case particles would only bedeposited in between the charged regions.

Furthermore, this embodiment may be adapted so as to use instead ofparticles in an aerosol, colloidal particles from the liquid phase,which will also be attracted by the charge patterns.

Other methods of bringing the particles close to the surface, which donot rely on an macroscopic electric field, may be used, e.g., inertialimpaction or thermophoresis.

Whilst the creation of charge patterns can easily be demonstrated forinsulating surfaces, the method may also be used for semiconductor andmetal surfaces, although the amount of charges and the time duration ofcharges might be smaller as compared with insulating surfaces.

An additional feature is that the substrate itself or the active surfacelayer can be very thin—just a few nanometers, for example 50 nm. Thisfacilitates the creation of a charge pattern on one side while theparticles are deposited on the other side of a substrate. This mayenable the stamping apparatus shown in FIG. 2 to be incorporated in thedeposition chamber shown in FIG. 3 in that the sample may be held inplace within the chamber and the stamping apparatus is brought againstthe underside of the sample to impart a pattern of electric charge (theupper surface of the electrode may form the stamp). This charge willthen be effective to attract aerosol particles streaming down onto theupper surface of the sample. The thickness of this substrate is limitedsimply by the dielectric constant of substrate material, the number ofcharges stored in the surface and the electric mobility of theparticles, which itself is a function of the particle size, the numberof charges on the particles and the medium in which the particle issuspended. Thus, thin foils could be used as substrate materials.

In order to generate electrically charged particles for depositing onthe substrate surface in the apparatus as shown in FIG. 3, the apparatusof FIG. 4 is employed in this embodiment. This is an aerosol generatorcapable of producing an aerosol with a volume flow of 1680 cm³/min and aparticle concentration of around 5×10⁵ cm⁻³ was used for particlegeneration. In FIG. 4, a furnace F1 generates metallic particles bysublimation. An electrical charger C1 is placed after the furnace tocharge the aerosol particles. Size selection takes place in adifferential mobility analyser DMA1. DMA apparatus exploits the factthat the electrical mobility of singly charged particles is amonotonically increasing function of particle size. While sending a flowof electrically charged particles in a perpendicular electric field, thefield causes particles to be attracted to one capacitor plate. Particleswith higher electrical mobility will be precipitated on the nearestportion of the plate and those with lower mobility will be carried alongwith the main flush flow. Only those with the correct mobility, andhence particles size, will be attracted to the facility of the samplingslit where they are swept out by the gas stream flowing through theslit. The DMA can produce particles with a closely controlled dimension,to within a standard deviation of a few percent. These particles areconducted to further furnace F2 where they are mixed with a hydride gasin order to produce further particles of a further composition. Theseparticles are subject to a close dimensional control in a further DMA2.For a given particle diameter, a distribution of diameters of ±0.2 ofthe diameter is achieved. The diameter of the particles may be as smallas 5 nm, or even molecular size. These particles are conducted to adeposition chamber DC, which is as shown in FIG. 3. An electrometer E1and a pump Pu are connected to measure the particle concentration and tocreate a gas flow therein for flowing the particles into the depositionchamber or precipitator.

The carrier gas is ultra pure nitrogen at ambient pressure and roomtemperature. Due to the generation process, the particles carry eitherone positive or one negative charge. For deposition, the aerosol flowsinto the apparatus shown in FIG. 3. A stagnation point flow wasestablished over the substrate. An electric field guides the chargedparticles towards the substrate surface where they are deposited. In theabsence of this field, the particles follow the streamlines of thecarrier gas and no deposition occurs. Thermal wet oxidised silicon (111)with oxide thickness of 500 nm and a plane surface were used assubstrates. The silicon was p+ doped with 0.01 to 0.02 Ω-cm resistivity.No special cleaning process was carried out except that of removingcoarse particles by blowing with nitrogen.

As an alternative to using a stamp, contact charging of the substratesurfaces may be carried out with a stainless steel needle that is slidover the substrate surface without applying pressure, both needle andsubstrate being earthed

For sample evaluation, scanning electron microscopy (SEM) was used toobtain the particle arrangement on the substrate surfaces.

FIG. 5 shows the homogeneous distribution of particles that is obtainedafter deposition of negatively charged, 30 nm indium particles in thehomogeneous electric field of the electrostatic precipitator with 150kV/m electric field strength. The particles are attracted to the surfaceby the macroscopic electric field. Their macroscopic distribution ishomogeneous over the whole sample area. The same behaviour is obtainedfor the deposition of positively charged particles in a negative field.

When negatively charged particles are deposited at 150 kV/m on asubstrate, which has previously been patterned with lines of negativesurface charges, as shown in FIG. 6, particle-free zones show up withinthe homogenous particle distribution. FIG. 6 is a scanning electronmicrograph that shows that these zones are approximately 10 μm in width.At their border is a narrow transition region (shown in the inset) witha width of around 300 nm in which the particle density increases fromzero to the mean density found on the rest of the substrate. The patternof particle free zones observed after the deposition corresponded to thepattern applied with a steel needle. This indicates a negative chargingof the contact area between substrate and needle.

In the case of deposition of positively charged particles with ahomogeneous electric field of ˜150 kV/m on a negative charged substrate,particles are deposited as shown in FIG. 7 in an approximately 10 μmline 80 lying in the centre of a 200 μm particle free zone 82. Theparticle density within this line 80 is higher by a factor ofapproximately 5–10 compared to the mean density on the rest of thesample. Again, the borders of the different areas are very sharp. Thelines were situated where the steel needle traversed the substrate. Inregions 84, remote from line 80, deposition by electrostaticprecipitation occurred.

For the fabrication of microelectronic components, it is often desirableto cover certain areas of a substrate selectively with a singlematerial, such as gold, while the rest has to remain clean. This meansthat it is preferable to avoid the uncontrolled coverage of thesubstrate caused by the electric field of the electrostaticprecipitator. Depositing positively charged particles on a chargedsubstrate with the electrostatic precipitator turned off, i.e. noelectric field applied, the surprising result is that the amount ofcharges on the substrate is sufficient to attract the particles from thegas flow. This means that the deposition becomes very selective and onlythe parts of the sample that are charged will be covered with particles.Line width of approximately 10 μm can be achieved. When the same processwas carried out with negatively charged particles no particles at allwere deposited.

When handling the substrate under ambient conditions the surface willhave a contamination layer consisting mainly of water. During thecontact electrification, charges are trapped in the silicon oxidesurface as well as in the contamination layer. The latter are mobile andcan move within the contamination layer. This leads to a broadening ofthe charge patterns on the surface. Using surfaces without thiscontamination layer improves the sharpness of the boundary betweencharged and non-charged regions. As preferred therefore, measures andmeans are employed to remove or prevent the formation of the watercontamination zone, such as heating the substrate in a water-freeatmosphere.

In one specific embodiment, silicon with a 1 μm layer of oxide waspressed against a Compact Disc (CD) master. A CD master is a metal platewith protrusions corresponding to where the depressions in the CD willbe. These protrusions are on the scale of 1 μm. The result after aerosolparticle deposition is shown in scanning electron micrographs (FIGS. 8 ato c). This demonstrates that particles gather on the contactelectrified spots. FIG. 8 a is the case for a uniform deposition with nolocally charged regions. FIG. 8 b, on the same scale as FIG. 8 a, showsthe deposition on locally charged regions by pressing with the CDmaster. FIG. 8 c is a reduced scale view of FIG. 8 b.

In accordance with a specific embodiment of the invention, as shown inFIG. 9, lines were made by gently sliding a metal needle against aninsulating (SiO₂) surface. This shows that it is possible to collectpositively or negatively charged particles with or without applying anexternal electric field in the precipitator. A resolution below 50 nmmay be attained.

It is possible to cover a surface with nanometer resolution withsubstances that can be electrically charged and dispersed in a carriergas. The size range of the building blocks ranges from several hundredsof nanometers down to individual molecules. The flexibility of thisprocess permits the creation of structures with resolutions from themillimeter size range (e.g. sensors) down to the 100 nm or even lowersize range (e.g. quantum devices). This makes the connection between themacroscopic and the nanoscale world possible in one process step.Another result that could be observed is that it is possible to arrangeparticle chains of different particle densities closely beside oneanother.

For the fabrication of electronic nanostructures, it is desirable thatthe charging process should neither destroy nor contaminate thesubstrate surface. Provided that one chooses the correct materialcombination, e.g., a sufficiently hard material is pressed against asofter surface, then the surface will elastically deform withoutpermanent deformation, provided the contact pressure is sufficientlylow. A hard material will not damage the substrate since the harmlesscontact is just sufficient to create the charge pattern and no forceswill be applied. Actually, creating surface defects, e.g. scratches,will ruin the effect of contact charging. With a softer material, i.e.where the bonds between the surface and the bulk atoms are not strong,it is possible that material might remain on the surface after thecontact.

The limitations for structural resolution of the method for fabricatingdistinct structures on a surface are mainly given by the number ofcharges stored in the surface, the number of particles deposited, andthe electrical mobility of the particles. The electrical mobility is afunction of the particle size, the number of charges carried by theparticle and the medium the particle is suspended in.

This invention finds particular application in circumventing the limitsof conventional photolithography. As circuits get ever smaller, thenumber of layers of metal lines (called vias) used to connect thedevices on the chip increases, becoming one of the largest components ofthe cost of chip manufacture. Each layer of metal requires a separatelithographic step, where photoresist is applied, exposed, and developed,followed by evaporation of metal, and finally lift-off of excess metal.Here, it permits the fabrication of leads with nanometer dimensionwithout any lithography step and without destroying the underlyingstructure.

Even the subsequent deposition of different material or differentmaterial sizes is possible by first creating a charge pattern anddeposition of one sort of particles followed by a second charge patterncreation and another particle deposition. Here, a fixation step for theinitially deposited particles might be necessary, such as annealing.

The present invention may also be utilised to replace the very finelithography employed in making chemical or biological sensors. It mayalso be used for fabricating catalytic structures.

Optical detectors with sub-picosecond response times have been made with(very slow) electron beam lithography and metallisation. In this way,interdigitated Metal-Semiconductor-Metal junctions are formed withlateral metal-metal spacing of below 50 nm. With method of thisinvention, an entire optoelectronic device may be fabricated veryefficiently, such as optoelectronic components based on nanoparticles.For some of these, ordering of the particles on the scale of thewavelength of the light is crucial. Among such components are quantumdot based laser and light emitting diodes.

The method may also be used in photonic bandgap materials—particlesplaced in arrays ordered on the scale of the wavelength of light whichexhibit a band gap for the photons, so that some wavelengths are notpermitted to pass. This has applications in optical communication.

The invention also finds application in the fabrication of interferencecolouring and anti-reflective coatings and for the construction ofnanostructured surface, which exhibit unique tribological properties,such as wear resistance.

Further application could be found in the fabrication of magneticstorage devices, flash memory devices, electroluminescence displays.Also for the controlled seeding of the growth of nanotubes andnanowhiskers, the present invention can be applied.

Additionally, projecting neutral particles with a higher speed towardssurface regions charged by the method would lead to a charge transferfrom the surface to the particles permitting particles scattered by thesurface to acquire a charge.

The invention also finds application in the removal of particles from agas or a liquid.

Referring now to FIG. 10, this shows examples of Coulomb—blockadedevices created by the method in accordance with the invention. FIG. 10a shows chains of nanoparticles 100 forming wireless single-electronlogic based on electrons hopping between nanoparticles. FIG. 10 b showssingle electron transistor structures with central nanoparticles 100,102 influenced by electrodes 104, 106.

FIG. 11 is a schematic diagram of the method in accordance with theinvention applied to fabricating nanometer-size metallic circuitstructures. A stamp 110 (shown conceptually) having a predeterminedshape 112 is pressed against a substrate 114 to create a correspondingpattern of charged regions 116. Metallic particles 118 of an oppositecharge type are then deposited on the substrate to adhere to the pattern116. After an annealing step, the particles merge to form continuousmetallic features 119.

Referring now to FIG. 12 this shows creation of multi-metallic surfacestructures by controlled nanoparticle deposition in accordance with theinvention. A stamp 120 is pressed against a substrate 122 to form apattern of charged regions 124 on the substrate, on which oppositelycharged nanoparticles 125 are deposited. The particles are then fixed onto the substrate by a suitable process. A further stamp 126 with adifferent stamp pattern is pressed against substrate 122 to produce asecond pattern 128 of charged regions. This permits nanoparticles 129 ofa different type to land on the second charged region.

Referring to FIG. 13, showing the fabrication of quantum dot lasers bythe method in accordance with the invention, wherein stamp 130 withmetallised protrusions 132, having dimensions of less then 20nanometers, is pressed against a substrate 134. The material of thesubstrate is in this example the n-type part of a laser structure;alternatively the substrate may constitute the p-type part of the laser.The metallised protrusions create charged regions or spots on thesubstrate to permit particles 136 to be deposited on the localisedcharge regions to create a pattern of n-type laser-active quantum dots138. After epitaxial overgrowth of the particles as at 139 with a p-typesubstrate to create the laser structure, the system is ready for finalprocessing.

The procedures of FIGS. 12 and 13 may be essentially combined, in thatrepeated operations of depositing p- or n-type particles, forming partsof laser structures, can be carried out, each operation employingparticles of a different diameter, and hence laser characteristics, e.g.wavelength. Finally, an epitaxial overgrowth is carried out, as in FIG.13.

Referring now to FIG. 14, there is shown the fabrication of photonicband gap materials by a method in accordance with the invention whereina stamp 140 having metallised protrusions 142 with lateral dimensions ofthe order of one quarter of the wavelength of the light in question (forexample about 10 micrometers) is pressed against a substrate 144 tocreate charged regions of a similar pattern. Micrometer sized particles146 are then deposited on the substrate to accumulate on the locallycharged regions. By prolonged deposition, particles will land on top ofeach other to create filaments 148 or chains of particles. This createsfilaments in a desired lattice structure having dimensions of the orderof the wavelength of light, whereby to create, by Bragg reflection,photonic band gaps for transmitted light.

Referring to FIG. 15, there is shown the fabrication of nanotube arraysby the method in accordance with the invention, wherein a stamp 150 withprotrusions 152 having lateral dimensions less than 20 nanometers ispressed against a substrate 154 to create locally charged regions.Nanometer sized particles 156 of the opposite charge type deposited onthe substrate to adhere to the locally charged regions. Using thenanoparticles as seeds, arrays or filaments of carbon nanotubes 158 canbe grown by chemical vapour deposition methods. This has application infield emission applications.

Referring to FIG. 16, there is shown the fabrication of nanorod arraysin accordance with the invention, wherein a stamp 160 with protrusions162 having lateral dimensions of less than 20 nanometers is pressedagainst a substrate 164 to create locally charged regions. Nanometersized particles 166 are deposited on the locally-charged regions, andthese particles are used as seeds to create filaments or nanorods, forexample semiconducting or magnetic materials, which are grown bychemical vapour deposition methods.

Referring to FIG. 17, there is shown a method of electrically chargingof aerosol particles. A stamp 170 having metallised protrusions withlateral dimensions of the order of centimeters 172 is pressed against asubstrate 174 to create a charged region 176. Neutral aerosol particlesare directed to the surface at high speed, thus rebounding on thesubstrate and taking away respective charged units from the chargedregion 176. Alternatively the substrate may not be electrically charged.The aerosol particles are nevertheless effective to “extract” electricalcharge from the substrate by the impaction process.

Referring to FIG. 18, there is shown a method for removing sootparticles from exhaust gas streams wherein a cylinder 180 of insulatingmaterial, positioned in an exhaust pipe, of for example an engine, has arotating metallic brush 182 mounted centrally on pipe 180 by supports184. The metal brush has metal filaments 186 contacting the inner wall,and as it rotates it charges the inner surface of the wall with negativecharge. The cylinder may be of silicon oxide, glass or ceramic. At anearlier stage, particles in the exhaust gas are positively charged,either as the result of the combustion process or by a separate meanssuch as a charger. These charged particles are then deposited on thecylinder wall. The brush functions to wipe the particles from the wallinto a exhaust channel 188 for further treatment.

Referring now to FIG. 19, there is shown a method of seeding epitaxialself-assembled dots for two and three dimensional arrays of quantumdots, by the method in accordance with the invention. As shown in FIG.19 a locally charged regions 190 are created by the method in accordancewith the invention. Using an epitaxial method, self-assembled dots 192are formed on the locally charged dots. The epitaxial method may bemolecular beam, a chemical beam or metal-organic vapour phased epitaxy,or any combination thereof. An insulating layer 194 is then grown overparticles 192, and this process is repeated to create three dimensionalarrays 196 of quantum dots.

In FIG. 19 b the method is somewhat similar to that shown in FIG. 19 aand similar parts denoted by the same reference numeral. However, in aninitial step nanoparticles, for example tungsten, 191 are deposited onthe electrically charged arrays by the method in accordance with theinvention. Using an epitaxial method, a thin buffer layer 194 is grownto cover the particles. In the next epitaxial step, self-assembled dots192 are formed on top of the embedded particles as described in the restof FIG. 19 a. A prolonged epitaxial process will create threedimensional arrays 196 of quantum dots.

Referring to FIG. 20, a flash memory structure comprises a substrate 200with source and drain electrodes 202. A gate structure 204 overlies aconducting channel 206. The gate structure comprises an oxide layer 208,a nanoparticle layer comprising nanoparticles 210 in an epitaxialovergrowth 212, and a further oxide layer 214, with a final metallicgate electrode 216. The nanoparticles 210 have the capacity for chargestorage, and may be of any suitable material. In the formation of thestructure, the oxide layer 208 is initially grown over the surface ofthe substrate, and then the nanoparticles are applied by the stampprocess, as above. Successive further steps of epitaxial overgrowth andselective etching create the structure shown.

1. A method comprising the steps of: forming one or moreelectrically-charged regions of predetermined shape on an insulatingsurface of a solid first material by contacting portions of said surfacecorresponding to said one or more regions with a solid material of atool so as to transfer electric charge from said tool to said surface;and permitting particles of a second material to flow in a vicinity ofsaid one or more electrically-charged regions, to interact with said oneor more electrically-charged regions.
 2. A method according to claim 1,wherein said particles are of nanometric dimensions.
 3. A methodaccording to claim 1, wherein the second material is different from thefirst material.
 4. A method according to claim 1, wherein said one ormore electrically-charged regions are charged with a charge of a firstsign, and said particles are charged with charge of a second sign, thesecond sign being opposite to that of the first sign.
 5. A methodaccording to claim 1, wherein said one or more electrically-chargedregions are charged with a charge of a first sign, and said particlesare charged with a charge of a second sign, the second sign being thesame as the first sign.
 6. A method according to claim 4, wherein saidparticles each carry one or more electric charges.
 7. A method accordingto claim 1, wherein an electric field is provided in a direction towardssaid surface so as to enhance the flow of said particles towards saidsurface.
 8. A method according to claim 7, wherein the electric fieldinduces a charge polarisation of said particles, which is effective todeposit said particles on said surface.
 9. A method according to claim1, wherein said particles are deposited on said surface in areasdetermined by said one or more electrically-charged regions in order tofabricate a structure.
 10. A method according to claim 9, wherein saidparticles are deposited on said one or more electrically-chargedregions.
 11. A method according to claim 9, wherein said particles aredeposited on said surface on areas other than said one or moreelectrically-charged regions.
 12. A method according to claim 1, whereinsaid particles are electrically neutral, and are projected against saidone or more electrically-charged regions to absorb electrical charge,and to rebound from said surface in an electrically-charged condition.13. A method according to claim 1, wherein the tool comprises a stamphaving a contoured surface with protrusions which is contacted with saidsurface of the first material and which has a configuration conformingto said portions of said surface of the first material.
 14. A methodaccording to claim 1, wherein the tool comprises an elongate objectwhich is pressed against said surface of the first material and drawnacross said surface in a desired path to define said one or moreelectrically-charged regions.
 15. A method according to claim 1, whereinthe tool is a tip of a scanning probe microscope.
 16. A method accordingto claim 1, wherein said surface is prepared so as to have nosignificant water or other conductive contamination.
 17. A methodaccording to claim 1, wherein said particles are arranged to flow as anaerosol.
 18. A method according to claim 1, wherein said particles arearranged to flow as a suspension in a liquid.
 19. A method according toclaim 1, wherein the second material is metallic, and the particles aredeposited on said surface of the first material and subsequentlyannealed to form metal seeds, and further comprising growingsemiconductor filaments or nanorods on the metal seeds by a chemicalvapor deposition method.
 20. A method according to claim 1, wherein in asecond stage of the method, said steps are repeated with respect to adifferent one or more electrically-charged regions of a differentpredetermined shape or size.
 21. A method according to claim 1,including a further step of epitaxial overgrowth of said particles ofthe second material, and wherein the first material, said particles, andmaterial of said epitaxial overgrowth form a laser structure.
 22. Amethod according to claim 21, wherein additional particles are depositedin subsequent steps, each step using particles of different lasercharacteristics.
 23. A method according to claim 1, including depositingsaid particles on said surface of the first material for a sufficientperiod to create filaments of the second material upstanding from saidsurface.
 24. A method as claimed claim 1, including providing a voltagebias between said tool and said surface.
 25. A method comprising thesteps of: (a) forming one or more electrically-charged regions ofpredetermined shape on a surface of a first material by contactingportions of said surface corresponding to said one or more regions witha tool so as to transfer electric charge from said tool to said surface;(b) forming on said one or more electrically-charged regions, selfassembled dots by an epitaxial method; and (c) growing an intermediatelayer on the self-assembled dots, and repeating said steps (b) and (c) adesired number of times.
 26. A method comprising the steps of: formingone or more electrically-charged regions of predetermined shape on asurface of a solid first material by contacting portions of said surfacecorresponding to said one or more regions with a solid material of atool so as to transfer electric charge from said tool said surface; andpermitting particles of a second material to flow in a vicinity of saidone or more electrically-charged regions, to interact with said one ormore electrically-charged regions, wherein the tool comprises a stamphaving a contoured surface which is contacted with said surface of thefirst material and which has a configuration conforming to said portionsof said surface of the first material.
 27. A method according to claim26, wherein said particles are of nanometric dimensions, and theparticles are subsequently annealed to form metal seeds, and furthercomprising growing semiconductor filaments or nanorods on the metalseeds by a chemical vapor deposition method.
 28. A method according toclaim 26, wherein said surface is prepared so as to have no significantwater or other conductive contamination.
 29. A method according to claim26, wherein said surface is an insulating surface of a semiconductorsubstrate and the contoured surface has protrusions.
 30. A methodaccording to claim 29, wherein said insulating surface is an oxidesurface.
 31. A method comprising the steps of: forming one or moreelectrically-charged regions of predetermined shape on a surface of asolid first material by contacting portions of said surfacecorresponding to said one or more regions with a solid material of atool so as to transfer electric charge from said tool to said surface;and permitting particles of a second material to flow in a vicinity ofsaid one or more electrically-charged regions, to interact with said oneor more electrically-charged regions, wherein the method furtherincludes a step of epitaxial overgrowth of said particles of the secondmaterial, and the first material, said particles, and material of saidepitaxial overgrowth form a laser structure.
 32. A method according toclaim 31, wherein additional particles are deposited in subsequentsteps, each step using particles of different laser characteristics. 33.A method according to claim 31, wherein said surface is an insulatingsurface of a semiconductor substrate.
 34. A method according to claim33, wherein said insulating surface is an oxide surface.
 35. A methodcomprising the steps of: forming one or more electrically-chargedregions of predetermined shape on a surface of a solid first material bycontacting portions of said surface corresponding to said one or moreregions with a solid material of a tool so as to transfer electriccharge from said tool to said surface; and permitting particles of asecond material to flow in a vicinity of said one or moreelectrically-charged regions, to interact with said one or moreelectrically-charged regions, wherein said particles are deposited onsaid surface of the first material for a sufficient period to createfilaments of the second material upstanding from said surface.
 36. Amethod according to claim 35, wherein said surface is an insulatingsurface of a semiconductor substrate.
 37. A method according to claim36, wherein said insulating surface is an oxide surface.
 38. A methodaccording to claim 1, wherein said insulating surface is formed on asemiconductor substrate.
 39. A method according to claim 38, whereinsaid insulating surface is an oxide surface.
 40. Apparatus comprising: atool having a portion of solid material constructed to contact one ormore portions of predetermined shape on a surface of a solid firstmaterial in order to transfer electric charge from said tool to saidsurface and thereby form one or more electrically-charged regions onsaid surface; and a system constructed to permit particles of a secondmaterial to flow in a vicinity of said one or more electrically-chargedregions, to interact in a predetermined manner with said one or moreelectrically-charged regions, wherein the tool comprises a stamp havinga contoured surface, for contacting said surface of the first material,with a configuration conforming to said one or more portions of thepredetermined shape.
 41. Apparatus according to claim 40, including anaerosol device which produces said particles so as to have nanometricdimensions.
 42. Apparatus according to claim 41, wherein said aerosoldevice is arranged to electrically charge said particles and wherein thetool comprises a stamp having a contoured surface with protrusions. 43.Apparatus according to claim 40, including a device constructed toproduce an electric field in a direction towards said surface so as toenhance the flow of said particles towards said surface.
 44. Apparatuscomprising: tool means for forming one or more electrically chargedregions of predetermined shape on a surface of a first material bycontacting portions of said surface corresponding to said one or moreregions; and means for permitting particles of a second material to flowin a vicinity of said regions, to interact in a predetermined mannerwith said regions.